The interaction of light and a conductor leads to collective oscillations of mobile/free charge carriers of the conductor under an incident electromagnetic field. Theses collective oscillations of electrons in a medium with mobile/free charge carriers are referred to as surface plasmon polaritons (SPP), or plasmons. The oscillations together with the induced charge-distribution generate a near-field effect that enables light to be concentrated below the diffraction limit and guided along a dielectric-conductor interface at optical subwavelengths and at frequencies in the hundreds of terahertz (THz).
Plasmon-based devices (also referred to herein as “plasmonic devices”) provide an opportunity to merge optics and electronics at subwavelength dimensions, offering the potential of integrating optical components with dimensions much closer to state-of-the-art electronic devices than photonic building blocks. When compared with photonic building blocks, plasmon-based devices enable increased bandwidth for short- and long-range communication, electro-optical or all-optical switching, and can significantly reduce cost and complexity due to the monolithic integration of electronic and optical components. Plasmonic structures also enable the concentration and confinement of optical fields to nm3 sized volumes. This allows for increased interaction between the optical field and optically active materials in a volume, enabling spectroscopy of analytes at sensitivities approaching the single-molecule level. Robust devices for detecting chemicals at this sensitivity level allow for a large range of applications, ranging from quality control in health and medical fields, to security and environmental monitoring.
While being important to applications of plasmonics, and providing beneficial near-field effects, the small size of plasmonic devices and the need for their close arrangement poses challenges for the implementation and application of such structures. Although bottom-up techniques based on nanoparticles or nano-rods can be utilized for the fabrication of sensing elements, tight size tolerances, missing methods for accurate placement on true nanometer accuracy levels, and non-tailorable dimensions to match resonant conditions, limit the usefulness of these bottom-up techniques, even for simple device geometries. Advances in top-down lithography have enabled the direct pattering of such structures to the extent that the limiting factors are now line-edge roughness (caused by pattern transfer) and material intrinsic inhomogeneity in the morphology, such as, for example, grain boundaries and domains.
Non-ideal geometries, high line-edge roughnesses and heterogeneous morphologies can lead to considerable losses, severely limiting the application range of plasmonic devices. Standard methods to increase, for example, grain size, do not lend themselves easily to the fabrication flow, as they cause significant surface roughness, which is a source of loss in itself. While enhancing the material and patterning properties of resulting devices is an important issue, a further challenge is the alignment of structures, for example, channels to the feedgap of a nanoplasmonic devices, requiring nanometer or even sub-nanometer tolerances.